The present invention relates generally to ultrasonic distance measuring systems. More particularly, a capacitance type transducer is disclosed that maintains substantially constant transducer diaphragm tension.
Capacitive (Sell-type) ultrasonic transducers are well known to the prior art. The transducers typically incorporate textured backplate (piston) that includes an electrically conductive front surface. A diaphragm that includes an insulating dielectric layer and a conducting layer is stretched over the backplate and attached thereto by either an adhesive or a mechanical clamp. The diaphragm may be arranged to mechanically respond to pneumatic and electrical forces. However, the mechanical response of the diaphragm (i.e. its operating frequencies and its amplitude) is dependent upon the diaphragm's tension.
A significant problem with capacitance type ultrasonic transducers is that the diaphragms tend to flow or creep under the influence of a tension, which in effect results in the sagging of the diaphragm and a corresponding loss of tension. The creep is a permanent deformation or elongation of the diaphragm even though it is not stressed above its elastic limit. Creep occurs at relatively low stresses and temperatures in the flexible materials typically used as diaphragm components such as gold and Kapton film. It is particularly noticeable in transducers operating in higher temperature environments. Our experience has been that if creep is uncompensated for, 10-15% of all transducers fail due to lack of diaphragm tension. Several attempts have been made to design transducers that automatically compensate for diaphragm expansions. One considered approach is to use a highly elastic material as the diaphragm itself. However elastic diaphragms have relatively short lives and are limited to low tension and frequency applications. A second approach is to spring load the backplate against a restrained diaphragm. For example, in U.S. Pat. No. 4,440,482 Shenk discloses an acoustic transducer assembly that uses a leaf spring to press the backplate into diaphragm tensioning engagement with the insulative layer of the diaphragm. Similarly, U.S. Pat. No. 4,081,626 discloses an alternative spring arrangement that urges a floating backplate into engagement with the diaphragm to maintain tension in the event of long-term plastic flow in the diaphragm.
Spring biased floating backplate arrangements are mechanically convenient and are effective at higher tensions and frequencies. However, they have several drawbacks. Most notably, as creep occurs, the position of the backplate relative to a target actually moves. This is problematic in many different situations such as when the measured distances are short, the system has very fine resolution and/or a reference target is being used. Modern transducers are capable of measuring distances to precisions on the order of one thousandth of an inch. Therefore, even small movements of the backplate due to creep compensation can significantly degrade the performance of the transducer. Another drawback of a movable backplate is that backplate misalignment may occur due to uneven creep compensation. Such misalignment will attenuate the strength of a returning signal. Additionally, floating backplate arrangements make it virtually impossible to create a positive seal between the diaphragm and the backplate. Thus the harsh environments typically surrounding industrial applications lead to undesirably high failure rates. Contamination between the backplate and the diaphragm can promote electrostatic charge accumulation, mechanically alter the systems response, and form corrosion on the backplate surface. Any of these problems can alter the characteristics of the transducer and would reduce the overall transducer accuracy. Backplate corrosion acts much like a layer of dielectric material and can adversely alter the transducers characteristics.